Spontaneous immunological reactivity to mouse mammary tumor virus (MTV) in normal C3H/HeN mice infected (C3H+) or uninfected (C3H-) with MTV demonstrated that 14-20 week old C3H+ mice were most reactive in migration inhibition and macrophage cytotoxicity assays and had depressed lymphoproliferative responses associated with an adherent suppressor cell. Loss of immunological reactivity was seen in mice bearing large, progressively growing plasmacytomas or lung adenocarcinomas. 50% of breast cancer and some benign breast disease patients have indirect migration inhibition reactivity against MTV, quantitated in a sensitive indirect microagrose droplet migration assay. Lack of reactivity in some patients was due to adherent suppressor cells. Previous studies on immunological monitoring of breast cancer patients, performed during the early post operative period, have been found to correlate with subsequent clinical course. Patients with low or negative lymphoproliferaive responses to autologous tumor extracts, and those with normal or elevated responses in mixed lymphocyte cultures, had a significantly increased incidence of recurrence. Reactivity to autologous tumor antigens in migration inhibition assays also associated with poor prognosis.